PROJECT SUMMARY Homologous recombination (HR) is a major chromosome damage repair tool. By eliminating DNA double- strand breaks and other deleterious lesions, HR is indispensable for the maintenance of genome stability. In humans, defects in HR directly lead to cancer. This renewal project focuses on the roles of the tumor suppressors BRCA1, BARD1, BRCA2, and PALB2 in the HR-mediated repair of damaged chromosomes. Considerable progress has been made during the last funding period, leading to important advances in understanding HR mechanism. Capitalizing on our success, a variety of in vitro and in vivo studies will be conducted to delineate the multifaceted roles of the BRCA1-BARD1-BRCA2-PALB2 ensemble in the HR reaction that is catalyzed by the recombinase RAD51. Novel hypotheses regarding how the HR factors facilitate the presynaptic stage of the HR reaction and how cancer mutations affect this critical HR step will be tested. The results from this renewal project will continue to provide insights regarding the functions of key HR factors, and will allow us to formulate detailed models of chromosome damage repair and genome maintenance via the BRCA-PALB2 tumor suppressor network. Our studies have direct relevance to cancer biology and to the development of chemical compounds to evaluate the potential of HR pathway-targeted therapeutic strategies. Moreover, the research materials generated during the course of our studies represent an invaluable resource for the DNA repair and cancer biology research communities.